This invention relates in general to heat transfer apparatus and processes for evaporating, distilling, freezing, heating or cooling liquids, and more specifically, to an improved, low wear mounting arrangement and drive for a whip rod that orbits within a heat transfer tube.
When processing fluids, it is often required to transfer heat to or from the liquid using a heat exchange surface, typically one formed of sheet metal, and a second process fluid on the opposite side of the sheet metal that is at a different temperature than the liquid being processed. This heat transfer between fluids may serve to warm the process fluid or cool it, as in a glycol chiller commonly used in building air conditioning systems. It may also serve to change the phase of the fluid, as in the production of fresh water by boiling it from sea water, or the production of ice slurries by partially freezing water or a water solution. Ice slurries are useful, among other applications, for cold storage to reduce peak load power demands in building air conditioning systems and to provide refrigeration for food such as milk stored on a dairy farm for transport to a processing plant and fish catches stored on fishing vessels.
The size, and hence the cost, of a heat exchanger depends on the heat transfer coefficient, which reflects resistance to heat flow through a layer of a "hot" fluid, a heat exchanger wall separating the hot and cold fluids, a layer of a "cold" fluid, plus deposits forming on either hot or cold surfaces of the wall. For economic reasons, a substantial temperature gradient is required to drive the heat transfer through these resistances. This high gradient limits the energy efficiency of evaporators or freezers by either limiting the number of stages or imposing a higher lift on a vapor compressor.
U.S. Pat. Nos. 4,230,529 and 4,441,963 to the present applicant disclose an approach to solving these problems using a vertical, thin-walled, open-ended heat transfer tube (or tubes). These early patents teach driving the tube or tubes in an orbital or wobbling motion. This orbital tube motion increases the heat transfer efficiency by swirling a liquid to be evaporated into a generally thin film over the inner surface of the tube. This increases the evaporation surface area and decreases the thermal resistance by decreasing the thickness of the liquid layer. The orbital motion also aids in heat transfer into the tube at its outer surface produced by condensation of a heated vapor stream. The condensation increases the thickness of the liquid layer at the outer surface, and hence its thermal resistance. The orbital motion throws off the droplets, thereby increasing the heat transfer at the outer wall.
U.S. Pat. No. 4,618,399 describes an improved heat exchanger using a whip rod located in the tube which spreads the feed liquid into a highly thin and uniform film to reduce its thermal resistance and to enhance its evaporation. The whip rod also controls the build up of solid residue of evaporation. The rod revolves over the inner surface of the tube in response to an orbiting of the tube and the fluid in the tube. The '399 patent discloses several arrangements for mounting the rod in the tube, yet allowing it to orbit without rotation about its own axis. The include lengths of cables secured to the whip rod at its upper and lower ends, a flexible, but non-rotating anchor connected between a base and the lower end of the rod, and a double universal joint also connected between the lower end of the whip rod and the base. While the whip rod is effective as a film distributor, the mounting arrangements have disadvantages. They increase the overall material, assembly and operating costs. Also, they fail. Material fatigue of flexible cables supporting the whip rods is a particular concern.
U.S. Pat. No. 4,762,592 describes an orbital drive that overcomes the manufacture, assembly, wear and balance problems of the earlier eccentric-crank drives for the tubes and fluids and other structures that move in unison with the tubes. This improved drive uses a rotating counterweight or weights mounted on the evaporator and a spring-loaded strut suspension for the evaporator. The counterweights and the mass of the evaporator revolve around one another as the counterweights rotate. However, it requires the orbital movement of a large mass. This large mass increases the power requirements (particularly on start up), increases the demands on the spring-strut suspension, can lead to an early fatigue failure for the suspension, and generally increases the construction and operation cost of the system.
This approach of orbiting the tubes has been used first for evaporation and distillational and then for freezing. The orbital motion of the entire exchanger, for freezing and boiling applications, suffers from the problems noted above, as well as being difficult to adapt to moving base operation and creating a disquieting psychological impact.
Other problems arise when these orbital heat exchange units are scaled up to more commercially useful sizes and operated under conditions that maximize the heat transfer flux. A straightforward way to scale up at the desired surface-to-volume ratio is to use more tubes.
The masses of the tubes, or the containers, fluid, and tubes, place extreme strains on rotary bearings of eccentrics coupled between a rotary power source and the end application of the force. Large forces quickly produce wear in bearings and at drive surfaces causing play in the drive train and a loss of the desired phase relationships between the movement of groups of tubes.
U.S. Pat. Nos. 5,385,645 and 5,768,894 teach driving the whip rod directly at only one of its ends for any application, boiling or freezing. For example, together they disclose drive plates that (i) suspend a set of rods, (ii) capture the rods in holes in the drive plate to push the rods, while simultaneously using the holes to provide a process fluid inlet to the tube, and (iii) operate a set of cranks that each mount and rotate at least one rod in each tube. A special concern in ice-slurry applications is that at a high cooling rate, ice forming on the heat transfer tube can not only reduce heat transfer efficiency, but it can also grow to fill the tube with ice and eventually freeze the whip rod in the center of the tube. Also, there is typically more ice near the bottom of the tube than the top. It is therefore desirable to reduce the number and size of mechanical obstructions to the exit of the ice slurry from the bottom of the tubes. This suggests a top mounting of the rods for this application.
While these arrangements offer the various advantages discussed in these patents, it was found that there are persistent and significant wear problems due to friction. These problems appear as uneven wear on the rods as well as wear in bearings and other load bearing surfaces in the drive train. For example, in a present commercial form of the hanging whip rod of this type of heat exchangers, as shown in FIG. 1, the thrust load of the weight of the rods as they orbit is transmitted via ball-and-socket bearings to a drive plate, and then from the drive plate to a tube sheet which mounts the tubes (including three pairs of sliding motion bearing surfaces: rod head-to-bearing, bearing block to an orbiting drive plate, and orbiting drive plate to the fixed tube sheet). Some of the sliding motion may be very fast because the diameter ratio of the tubes and the rod moving in a planetary motion increases friction and wear in all the rod support and drive members. This wear limits the performance and life of the heat exchanger. The high friction also increases the power required to drive the rods.
It is therefore a principal object of this invention to provide support and drive for a hanging whip rod orbiting in a tube-type heat exchanger which significantly decreases friction and wear and correspondingly increases the operational life of the rods and the drive for the rods.
A further object is to provide a hanging whip rod support and drive with the foregoing advantages which reduces the power required to operate the heat exchanger.
Another object is to provide a rod support and drive which also acts a flow rate control mechanism for the fluid infeed to the tubes.
Still another object is to provide a hanging whip rod orbital drive that allows the rod to orbit freely as the orbital radii shrink down to zero, as in the case of a frozen tube.
Another object is to provide a rod support and drive with the foregoing advantages which has a favorable cost of manufacture, both in terms of a low part count and in terms of ease of assembly and ease of disassembly and repair for low cost maintenance.